Peptide Internal Standards for Absolute Protein Quantification
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Solution Overview
Problem
Current methods for protein quantification, such as Western analysis and ICAT technology, face limitations including reliance on specific antibodies, relative quantification, and inability to accurately measure low abundance proteins and post-translational modifications.
Innovation Solution
Development of peptide internal standards labeled with mass-altering labels for use in high-performance liquid chromatography (HPLC) with online detection by multistage mass spectrometry (MSn), allowing for absolute quantification of proteins and identification of modification sites like ubiquitination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If Western analysis is used for protein quantification, then protein detection can be performed, but it requires specific antibodies and provides only relative quantification
Solution Approach 1:
The invention uses stable isotope-labeled peptide copies of target peptides as internal standards. These synthetic peptide copies are chemically identical to endogenous peptides but contain stable isotope labels (e.g., 13C, 15N) that increase their mass. The labeled peptides are spiked into samples at known concentrations and co-elute with endogenous peptides during chromatography, allowing precise quantification through mass spectrometry without requiring antibodies
Solution Approach 2:
The invention replaces the immunological detection mechanism (antibody-antigen binding) with a mass spectrometric detection mechanism. Instead of using antibodies to detect and quantify proteins, the method uses mass spectrometry to detect and quantify stable isotope-labeled peptide internal standards and their corresponding endogenous peptides based on their mass-to-charge ratios, eliminating the need for antibody preparation and validation
2Productivity
If ICAT technology is used for protein analysis, then large-scale protein identification can be achieved, but quantification remains relative and specialized chemistry is required
Solution Approach 1:
The invention changes the chemical parameter of the internal standard by using stable isotope-labeled peptides instead of ICAT reagents. The stable isotope labels (13C, 15N) provide a mass difference parameter that enables detection by mass spectrometry without requiring affinity tags or specialized chemical groups. This simplifies the chemistry while maintaining high throughput capability
Solution Approach 2:
The invention extracts the quantification function from the protein analysis workflow by adding separate, known amounts of stable isotope-labeled peptide internal standards to samples. This separates the quantification step from the protein identification step, allowing each to be optimized independently and eliminating the need for complex ICAT chemistry while maintaining scalability
3Speed
If microcapillary LC-MS/MS is used for protein identification, then identification speed increases, but absolute quantification cannot be determined
Solution Approach 1:
The invention performs preliminary action by spiking stable isotope-labeled peptide internal standards into samples before any processing steps. These labeled peptides are present from the beginning at known concentrations and undergo the same extraction, digestion, and chromatography steps as endogenous peptides. This preliminary addition ensures that the internal standards experience identical sample preparation conditions, enabling accurate absolute quantification while maintaining the speed of LC-MS/MS analysis
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables accurate and absolute quantification of proteins and their modifications, including low abundance proteins and post-translational modifications, in complex samples, providing a rapid and high-throughput analysis of proteomes.
Implementation Method 1
analysis by mass spectrometry, wherein the presence of the bond indicates a site of ubiquitination
Implementation Method 2
automated, data-dependent electrospray ionization (ESI) tandem mass spectrometry (MS/MS)
Data Source
AI summary
The invention provides a method detecting and quantifying proteins by mass spectrophotometric analysis using peptide internal standards and provides a highly sensitive way of detecting protein modifications. In one aspect, the invention provides a method for determining a site of ubiquitination in a polypeptide and for evaluating ubiquitination targets in a population of polypeptides. In this way, a proteome ubiquitination map can be obtained which comprises information relating to the ubiquitination states of a plurality of cellular polypeptides. Maps can be obtained for a variety of different types of cells and cell states. For example, ubiquitination targets in normal and diseased cells can be evaluated. Preferably, the map is stored as data files in a database. Individual ubiquitinated polypeptides identified can be used to generate molecular probes diagnostic of a cell state and/or can serve as targets for agents that modulate one or more cellular processes.